Analysis of Potential Impacts of Climate Change and Deforestation on Surface Water Yields from the Mau Forest Complex Catchments in Kenya

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Abstract

This study focuses on an in-depth understanding of the extent to which climate change and
deforestation impact on the surface water yields in the Mau forest complex, the largest of the five
water towers in Kenya. The forest complex forms the source of important national and
international rivers including Sondu, Nyando, the Nile, Mara and Ewaso Ng‟iro which are
economic life lines for three sectors of the country‟s economy: Tourism, Agriculture, and Energy.
Demand for more arable land to support the area‟s growing population has led to large tracts
previously preserved as gazetted forests being excised since late 1940s which has affected the
hydrology of the water tower. The study analysed the past climate and forest cover changes and
modelled future changes in climate and the extent to which these changes impact on the surface
water yields in this region.
Potential impacts of climate change and deforestation on surface water yields were analysed using
a modelling approach in which observed and projected climate outputs from a regional climate
model, commonly referred to as PRECIS, which stands for Providing Regional Climate for
Impacts Studies, and changes in forest cover were used to drive a hydrologic model, the Soil and
Water Assessment Tool (SWAT). Outputs from SWAT were used to assess the impacts of
changes in climate and forest cover on the surface water yields from the Mau forest complex water
tower as exemplified by changes in river flow volumes. The study analysed historical climate,
forest cover and streamflow changes that have taken place within these catchments with a focus on
South West Mau forest block located within Sondu River basin. Projections of future climates
under the Special Report on Emissions Scenarios (SRES) A2 emissions scenario were obtained
from the third generation Hadley Centre Regional Climate model (HadRM3) using PRECIS
regional climate model while Landuse/Landcover (LULC) changes were obtained from
LANDSAT satellite image analysis using supervised classification methods.
It was shown from the analyses of historical data that the climate of the area has progressively
become warmer and wetter since the 1970s. Analysis of temperature and rainfall indicated
increasing trends while streamflow indicated a decreasing trend. Analysis of forest cover indicated
increasing deforestation trends over the Mau forest complex of about 27% between 1973 and
2010. Analyses of mean daily maximum and minimum temperatures indicate that days and nights
in this area have become warmer since the 1961-1990 baseline period by about 0.5˚C and 0.4˚C
respectively. In the same period monthly rainfall distribution has shown increasing trends in the
relatively dry DJF and SON seasons which have become wetter by about 7.5% and 9.2
respectively, and decreasing trends in the relatively wet MAM and JJA seasons which have
become drier by about 2.2% and 4.5% respectively. The changes in the distribution of monthly
rainfall translate into redistribution of seasonal water yields from the catchments.
Analysis of projected temperature and rainfall shows strong indications that the climate of the area
will significantly change in future under the SRES A2 emissions scenario with warmer and wetter
climates being experienced by 2030 and beyond. The annual average temperatures and rainfall are
expected to change by about 2.7˚C and 4.7% respectively by 2030, and by 4.7˚C and 18.9%
respectively by 2050, relative to the baseline. The projected monthly rainfall distribution show
increasing trends in the relatively dry DJF and SON seasons while showing decreasing trends in
the relatively wet MAM and JJA seasons.
Simulated water yields under climate change at the baseline forest cover scenario show an
increasing trend but show decreasing trends as forest cover over South West Mau forest block
diminishes. Projected water yields in 2010s and 2030s indicate a decreasing trend in potential
water yields of about 0.69 MCM/yr in 2010s and 0.71 MCM/ yr in 2030s
Results of the study indicate that indeed deforestation of the Mau forest catchment has notable
impacts on water yields from the water tower. The study has shown that the overall impacts of
climate change coupled with deforestation on the water yielding capacity of the Mau forest
catchments will be a reduction in the potential annual water yields in the range of 15% and 16%
per decade of the baseline yields to between 28% and 45% in 2010s and between 31% and 50% in
2030s respectively.
Results of this study have provided useful insights into the impacts of climate change and
deforestation on surface water yields which can be used to inform short, medium to long term
planning of water resources. The results, methods and products of the study should be
incorporated in the mainstream economic development strategies especially in the development of
the national water resources master plan. To ensure adequate flow in rivers from this important
water tower to sustain both the socio-economic and environmental uses, it is recommended that
efforts to rehabilitate the Mau forest complex be stepped up and sustained.